1. Field of the Invention
The present invention relates to an evaporative cooling power cable line capable of transmitting a large amount of electric current over a long distance.
2. Description of the Prior Art
The previous power cable lines have incorporated cooling systems generally by supplying various fluids, such as air, inert gas, water, insulating oil, etc. This cooling fluid is generally introduced into the interior or exterior of the cable under the effect of a force generating means, such as a fan or pump that sometimes utilizes the specific heat or evaporative latent heat. It has been found that in these conventional systems it is impossible to transmit a large current in the order of 8000 to 12,000 amperes over a long distance of, for example, 10 to 20 km.
Since the conventional system have generally utilized a form of rotary equipment, such as a pump or fan, it is often necessary to service them and police them with periodic inspection with the accompanying costs. For this reason, a cooling system that utilizes rotary equipment can not generally be utilized if a completely automatic system is desired.
While a conventional system may be capable of providing sufficient cooling efficiency in order to transmit between 2000 and 4000 amperes of current, it has been found that it is difficult to permit the flow of a large amount of refrigerant owing to the restriction in the size of the cable. Since the amount of pressure drop that exists for forcibly flowing refrigerant to cool the cable is generally proportional to the cube of the length of the cable and to the fourth power of the transmitting current, the capacity for carrying electric current is generally lowered in a long distance transmission line. For this reason, it has been found difficult to transmit a great deal of electric current such as 5,000,000 to 10,000,000 KVA which represents approximately 12,000 amperes at 500 KV cable line over a distance, for example, of 10 km.
It has been suggested to effectuate such a large power transmission over a long cable length that the cooling system be divided up in several sections of cable line, with rotary equipment and a cooler provided in each respective section. This, however, just further increases the cost and the reliance upon rotary equipment.
It has also been suggested that the amount of flowing refrigerant be lessened to reduce the pressure drop and that a cooler be provided adjacent the cable in such a manner that the refrigerant is taken out of the cable to be cooled and then is reintroduced into the interior of the cable insulating layer. However, generally the flowing speed of the refrigerant is small and the heat resistance of the boundary film between the refrigerant and the conductor becomes generally large so that the transmission of large current still remains difficult.
Generally, utilizing a forced cooling system, it has been found that only those systems that introduce the liquid refrigerant into the interior of the cable insulating layer or internal cooling system are capable of transmitting large amounts of current. The dependency upon rotary equipment, however, makes this system prone to failure. For example, if the rotary equipment is stopped, the cable temperature will increase in a relatively short time, causing a breakdown. In addition, if the current must be transmitted over a long distance, a forced cooling system must overcome a large pressure differential and, correspondingly, the rotary equipment, such as a pump, must be of large capacity and the cable also must be reinforced.